A novel EF-hand protein, CRACR2A, is a cytosolic Ca(2+) sensor that stabilizes CRAC channels in T cells

Orai1 and STIM1 are critical components of Ca(2+) release-activated Ca(2+) (CRAC) channels that mediate store-operated Ca(2+) entry (SOCE) in immune cells. While Orai1 and STIM1 co-cluster and physically interact to mediate SOCE, the cytoplasmic machinery modulating these functions remains poorly understood. We sought to find modulators of Orai1 and STIM1 using affinity protein purification and identified a novel EF-hand protein, CRACR2A (CRAC regulator 2A, EFCAB4B, FLJ33805). We show that CRACR2A directly interacts with Orai1 and STIM1, forming a ternary complex that dissociates at elevated Ca(2+) concentrations. Studies using siRNA-mediated knockdown and mutagenesis show that CRACR2A is important for clustering of Orai1 and STIM1 upon store depletion. Expression of an EF-hand mutant of CRACR2A enhanced STIM1 clustering, elevated cytoplasmic Ca(2+) and induced cell death, suggesting its active interaction with CRAC channels. These observations implicate CRACR2A, a novel Ca(2+) binding protein, highly expressed in T cells and conserved in vertebrates, as a key regulator of CRAC channel-mediated SOCE.